This invention relates to overvoltage protection for power supplies and illustrates its application specifically to overvoltage protection with a switched-mode power supply using current-mode control to provide output voltage regulation.
Current-mode control of switched-mode power supplies has been used for many years and has been frequently discussed in the literature available to one skilled in the art. Current mode control uses an inner control loop to control the peak inductor current in accordance with an error signal from an error amplifier. As the error signal changes, the peak switching current follows proportionally to deliver more or less current to the load as required to maintain output voltage regulation. Failures in the error amplifier section of the voltage control unit, its reference voltage, the flip-flop, or the comparator can cause the power supply's output voltage to exceed its prescribed or normal value. Some form of power supply overvoltage fault protection is generally required to protect the load against voltage overstress. One prior art approach has been to place a zener diode across the power supply's output to clamp the output voltage. Due to the limited power handling capabilities of practical zener diodes, the zener will generally fail shorted when subjected to a sustained power supply overvoltage fault condition. The load is thereby protected, provided that the power supply is either current limited or fused. Unfortunately, there is no load protection in the event that the zener fails open-circuited. Thus, sophisticated power supplies do not use zener diodes to provide overvoltage fault protection.
The overvoltage protection for some of the more sophisticated power supplies of the prior art has been to completely disable the power supply when an excessive output voltage has been sensed. This may be either latching or non-latching. Either of these prior art approaches have deficiencies, since the prior art latching approach may shut down a power supply in the face of temporary overvoltage conditions such as voltage surges, whereas the non-latching approach allows the overvoltage condition to be continued on a cyclic basis until manual intervention. Both of these approaches have used elements of the feedback control loop, where a failure may be the cause of the overvoltage condition, as part of the overvoltage protection circuit.
The present invention solves the above listed deficiencies by cyclically, but momentarily, shutting down the power supply upon detection of an overvoltage and only shutting the power supply down permanently if the overvoltage condition is maintained for more than a predetermined time.
It is thus an object of the present invention to provide an improved overvoltage protection scheme.